This invention relates to methods for testing viscoelastic materials to determine their rheological properties and to apparatus for performing such testing. More particularly, this invention relates to methods for applying forces to viscoelastic materials enclosed under pressure and measuring the behavior of the materials, and to apparatus for carrying out these methods.
Prior art methods and apparatus for testing viscoelastic materials include U.S. Pat. No. 2,037,529, relating to the Mooney Plastometer, wherein a plastic material is sheared between rotor and stator and the related forces are measured. In U.S. Pat. No. 3,681,980 a method and apparatus are shown wherein a sample is enclosed in a chamber and subjected to shearing forces by a biconical rotor contained therein. The behavior of a vulcanizing elastomer is thus measured at a fixed temperature, usually a standard temperature for vulcanizing rubber, between 150.degree. and 200.degree. C. U.S. Pat. No. 4,343,190 discloses a method and apparatus for measuring such vulcanization behavior by shearing a rubber sample between two dies, one of which is displaced with respect to the other, and no separate rotor is required.
Other methods and devices are used for testing the behavior of viscoelastic materials which are not in the process of curing or cross-linking. In such cases, the processability of a material may be examined by subjecting it to shearing forces which approximate those forces employed in mixing, shaping or forming operations. Generally, higher shear rates are employed for these so-called processability tests than those used for cure testing, and often the behavior of the material is examined at several different shear rates in order to give a complete picture of the behavior of the material under different process conditions.
Capillary rheometers are often used to measure properties of viscoelastic materials, and can subject a sample to a variety of shear rates so as to evaluate its behavior under processing conditions.
The known methods and apparatus for testing viscoelastic materials are able to test certain aspects of the properties of these materials, but a plurality of different tests are required on widely differing apparatus, and the correlation of the results is often difficult and unreliable. A need exists for a test method and apparatus for viscoelastic materials which could treat a single sample and produce fast, accurate data on the different properties of the viscoelastic materials.